Tuning fork arrangement for electromechanical oscillators



March 1970, c. F. CLIFFORD 3,

TUNING FORK ARRANGEMENT FOR ELEGTROMECHANICAL OSCILLATORS Filed April18, 1968 mlllll'l 1 Ceq/ Frank car wrewfw? United States Patent3,501,910 TUNING FORK ARRANGEMENT FOR ELECTRO- MECHANICAL OSCILLATORSCecil Frank Clifford, Newbridge Works, Bath, Somerset, England FiledApr. 18, 1968, Ser. No. 722,319 Claims priority, application GreatBritain, May 15, 1967, 22,329/67 Int. Cl. G04c 3/00 US. Cl. 58-23ABSTRACT OF THE DISCLOSURE A mounting arrangement for a single reed or atuning fork of an electromechanical oscillator in which an inertiaweight is directly attached to one end of the reed or the nodal point ofthe tuning fork, and the inertia weight is mounted on a spring which issupported by a base, the spring preferably being arranged so that itwill readily move in a direction in which out-of-balance forces of theoscillator will act, so that noise and other undesirable effects arereduced.

This invention relates generally to electromechanical oscillators, andmore particularly to a mounting arrangement for the mechanicallyoscillating member of an electromechanical oscillator.

It is becoming usual practice to make the oscillating member of anelectromechanical oscillator from a strip of metal which, if theoscillating member is a tuning form, is bent to the required U form andhas a support attached to it at its nodal point, that is to say, at thecenter of the curved portion of the U. A fork of this kind is simple toconstruct and light in weight and both these features are important in adevice which is required to be manufactured in quantities, in smallsizes and at the lowest possible cost. A further advantage, which isgained both in the reed type oscillating member and the tuning fork, isthat alloys having a substantially zero temperature coefficient ofelasticity are available in suitable strip form and this makes itpossible to provide a tuning fork which has a high degree of constancyin its frequency of oscillation despite variations in ambientconditions.

The reed or fork is, of course, provided with a pair of transducers, oneof which is connected in the input circuit and the other in the outputcircuit of an amplifier by which the reed or work is maintained incontinuous oscillation.

If an oscillating member of either kind is mounted directly on a base oron a clock movement which it is to control, any shocks to which the baseor clock movement may be subjected are transmitted to the oscillatingmember and may momentarily disturb its steady oscillation. A moreimportant circumstance is that the vibration of the oscillating memberis transmitted to the base or movement and may set up a volume of noisewhich is quite unacceptable, more especially if the noise is transmittedto a case in which the clock movement is mounted and the case acts as asounding board. There is another factor, of a technical nature, known asthe support effect. Certain losses occur due to all conventional typesof support and the effect of these losses is to reduce the Q of thefork, and usually, the frequency of oscillation. It has been found thatwhen the oscillating member is mounted on a support of a conventionaltype the support has a natural frequency of its own. Hence, when theoscillating member oscillates, the support tends to vibrate at its ownnatural frequency and a certain amount of pulling results. This changesthe oscillation frequency slightly. It has been established that thechange in oscillation frequency may be such that it makes as much as 20seconds per day difference in the speed of a clock controlled by theoscillating member.

5 Claims 1 3,501,910 Patented Mar. 24, 1970 The principal object of theinvention is to provide a mounting arrangement for an oscillating memberof the kind described which will prevent or substantially reduce thetransmission of noise or shocks and substantially reduce the supporteffect.

The invention consists of a mounting arrangement for the mechanicallyoscillating member of an electromechanical oscillator comprising aninertia weight which is attached to the oscillating member, a springattached at one 0 end to the inertia weight, and means by which theother end of the spring may be attached to a base on which the mountingarrangement is to be carried.

Where the oscillating member is a tuning fork the inertia weightispreferably attached to the fork substantially at the nodal pointthereof, and where the oscillating member is a reed the spring mayconveniently be constituted by an extension of the reed.

In order to facilitate the full understanding of the invention oneembodiment thereof will now be described with reference to theaccompanying drawings, in which:

FIGURE 1 is an elevation of one arrangement according to the invention;

FIGURE 2 is a side elevation of the arrangement of FIGURE 1;

FIGURE 3 is an inverted plan view of the same arrangement;

FIGURE 4 is an elevation of another arrangement according to theinvention;

FIGURE 5 shows the application of the invention to a compound reedoscillator;

FIGURE 5a shows a strip of metal from which the compound reed oscillatorof FIGURE 5 is made; and

FIGURE 6 shows the application of the invention to a simple reedoscillator.

Referring initially to FIGURES l to 3 of the drawings, a base on whichthe tuning fork arrangement is to be mounted is indicated at 11. Thismay in fact be one of the support plates for a clock movement which, isin turn, intended to be mounted in a clock case. A tuning fork 12 ismade from astrip of material which is bent into a U form and isadvantageously made from one of the nickel-iron alloys having asubstantially zero temperature co-efficient of elasticity such,'forexample, as the alloys known as Ni-Span C and Ni-Span D.

It will be understood that an amplifier is included and the tuning forkis provided with two transducers connected to the amplifier but theseare not shown since they are not a part of the invention and constitutewell-known art. The tuning fork 12 has attached to it, at or near itsnodal point, an inertia weight 15 which, as shown in FIGURE 2, issuspended just clear of the base 11. A support block 16 is mounted onthe base 11 by screws 17 and it has one end of a spring 18 attached toit by means of screws 19. The spring 18 contains a rightangled bend andits other end is attached to the inertia weight 15 by screws 20. As willbe clear from FIGURE 3 the centre part of the end portion of the springwhich is attached to the inertia weight 15 is cut away to permit thetine 13 of the fork to pass through it.

With this construction the major part of any vibration which istransmitted to the heavy inertia weight 15 is reduced by that weight andthe resilient mounting by means of the spring 18 ensures that theremaining vibration is further reduced. The mounting block 16 isconveniently made of a synthetic plastics material such as nylon whichhelps to absorb the remaining small amount of vibration.

In operation, each tine of the fork 12 generates a small amount ofcentrifugal force and an amount of inertial force as it oscillates, andthis tends to move the fork in the direction of the double headed arrow21. This tendency is transmitted to the heavy inertia weight 15 which isattached substantially at the nodal point of the fork. If acomparatively rigid mounting were used then the vibration due to thiscombined centrifugal and inertial action would be transmitted directlyto the 'base 11, resulting in the noise referred to previously.

By using the'heavy inertia weight 15 and arranging the spring 18 so thatit will yield in the direction of the arrow 21 it is ensured that thisvibration is not transmitted to the basell, or is only transmitted inheavily attenuated form. The support effect is also substantiallyreduced, if not eliminated. It was mentioned earlier that the supporteffect could alter'the rate of a tuning fork sufliciently to cause atime difference of as much as 20 seconds in one day. In such a case,mounting the fork in the manner described above can reduce this rateerror to as little as 2 seconds per day.

The improvement may be confirmed by a calculation, as set out below, inwhich certain assumptions have been madefor the sake of simplicity.Taking an arrangement in which the fork frequency is 300 HZ. and theinertia weight is such that it may be permitted to sag under its ownweight by 0.02 cm., assume that an RMS force of 100 grams is shaking thesupport. The force is equal to mass acceleration. Assume that the massof the inertia weight is also 100 grams. The acceleration a during onequarter cycle second i Low-1 cm/ ec/sec 1200 S 100 The distance moved in1 s1 1 m6 second (t) re s a t X -0.00034 cm.

where F is the fork frequency and F is the support frequency. In theabove example F is 300 and F is 35 so that the ratio ofreduction of theforce acting on the support is which is equal "to 0.0136. This ratiomultiplied by the force gives 0.0136 100=1.36 'grarns compared with 1.7grams given by the earlier calculationin which certain assumptions(which are not strictly correct) were made in the interests ofsimplicity. However, both calculations provide a good indication of theorder of the reduction. 7 Where the fork is oscillating at acomparatively low frequency, e.g. 30 c/s., the inertial force andcentrifugal force will both be very small and the main force tending tocause vibration to be transmitted to the base 11 will result fromout-of-balance tine forces which are nearly always present in somedegree. This will tend to act in the direction of the arrows 22 sincethe tines are vibrating in that direction, and in that case it may bedesirable to modify the spring 18 or its method of mounting so that thespring 18, or at least a major part of it, lies between the tines andparallel to them. Preferably the spring is exactly in the middle of thespace between the tines.

Such an arrangement is shown in FIGURE 4 in which an inertia weight 24is attached substantially at the nodal point to a tuning fork 23. Theweight is mounted on one end of a spring indicated by reference 25having a portion 26 which lies between, parallel to and equally spacedfrom the two tines. The continuing portion 27 of the spring lies atright angles to the portion 26 and the other end of the spring isattached to a support 28 attached by screws 29 to a base, not shown. Asbefore, the support 28 may be made of nylon or other material havingvibration absorbing properties. As an alternative, the element 28 may bea lug which is integral with the spring portion 27 and is bent over atright-angles to form a fixing lug.

FIGURES 5 and 5a show an arrangement in which the inertia weightaccording to the invention is used in conjunction with a compound reedoscillator consisting of a strip 32 (FIGURES 5 and 5a) which is formedwith two oppositely directed U shaped slots 51 and 52 (FIG- URE 5a) inorder to leave two centraltongues 53' and 54 which constitute secondaryreeds. The strip is then bent to a U form as shown in FIGURE 5, leavingthe ends, respectively 33 and 34, of the secondary reeds projecting. Thesecondary reed 33 has a magnet 35 attached to its free end while thesecondary reed 34 has a magnet 36 attached to its free end, the' magnetswhich is bent over toprovide means for fixing to a base with screws 41.

FIGURE 6 shows how the invention may be applied to a single reed. In theconstruction shown the reecl 42 is actually in one piece with the springby which the inertia Weight is supported and, as will be seen from thedrawing, the reed 42 has the inertia weight, which is in two parts 43aand 43b attached to it. The reed then. continues as the supportingspring with a right-angled bend, the part 44 beyond the right-angledbend being attached to a base 45 by means of screws 46. Two magnets(shown dotted), respectively 47 and 48, arecarried on the base 45 andare respectively surrounded by signal and drive coils 49 and 50.

I claim: v I 1. An arrangement for mounting a tuning fork member of anelectromechanical oscillator on a supporting base comprising an inertiaweight attached to said member at the nodal point of said member, aspring capable of being deflected in a first direction parallel to thelongitudinal direction of the oscillating member and in a seconddirection perpendicular to the said first direction and parallel to theplane in which the oscillating member oscillates, the spring beingattached at one end thereof to the inertia Weight, andmeans attached tothe other end of the spring by which the arrangement is mounted on thebase.

2. An arrangement as claimed in claim 1 in which the inertia weight liesbetween the tines of the fork, the said one end of the spring isattached to a face formed on the Weight at right angles to thelongitudinal direction of the tines, and the spring is formed with aright-angled bend. at a point part way along its length.

3. An arrangement as claimed in claim 1 in which the inertia weight liesbetween the tines of the fork, the said one end of the spring isattached to the weight so that the portion thereof nearest the weightlies parallel to the longitudinal direction of the tines, and the springis formed with a right-angled bend part way along its length.

4. An arrangement as claimed in claim 1 in which the tuning fork memberis a compound reed made from a strip of material formed with twooppositely directed U shaped slots defining two central tongues whichconstitute secondary reeds, the strip being bent into the form of a 5 6tuning fork to leave the ends of the secondary reeds pro- ReferencesCited jecting from the curved portion of the fork. FOREIGN PATENTS 5. Anarrangement as claimed in claim 1 in which the spring is formed by anextension of the reed, the inertia 11480980 5/1967 Franceweight beingattached to the strip forming the reed and spring part Way along itslength, whereby the part on 5 RICHARD WILKINSON Pnmary Examme one sideof the inertia weight forms the reed and the EDITH C. SIMMONS, AssistantExaminer part on the other side of the inertia weight forms the US. Cl.X.R.

spring. 31025

